It is required for a brake disc for a two-wheeled vehicle to have properties such as wear resistance, rust resistance, and toughness. Usually, wear resistance enhances as hardness increases. Meanwhile, since too high hardness may cause so-called brake squeal between a brake and a pad, brake the hardness of 32 to 38 HRC (Rockwell hardness C-scale) is required. Due to these demanded properties, a martensitic stainless steel sheet is used as a brake disc for a two-wheeled vehicle.
Heretofore, SUS420J2 has been quenched and tempered to control to the desired hardness thus fabricating a brake disc and, in this case, there was a problem that two heat treatment processes including quenching and tempering processes are required. Meanwhile, Patent Literature 1 disclosed an invention relating to a steel composition, which is capable of stably obtaining the desired hardness at a wider quenching temperature range than a conventional steel of a SUS420J2 steel, and is also used in the as-quenched state. This is the invention that the (C+N) content is reduced and also narrowing of the austenitizing temperature range caused thereby, in other words, narrowing of the quenching temperature range is compensated by the addition of Mn as an austenite forming element. Patent Literature 2 also discloses an invention relating to a steel sheet for motorbike disc brake, which is a low Mn steel and is used in the as-quenched state. This steel sheet is obtained by adding Ni and Cu as an austenite forming element which has the same effect as that of Mn, in place of reducing the content of Mn.
In recent days, it has also been required for a two-wheeled vehicle to reduce the weight of a vehicle body, and a study on reduction of the weight of a brake disc for a two-wheeled vehicle has been made. In this case, disc deformation due to disc material softening caused by heat generation at the time of braking will be a future problem, and there is a need to improve heat resistance of the disc material so as to solve such problem. One of solutions thereof is an improvement in temper softening resistance, and Patent Literature 3 disclosed an invention relating to a method for improving heat resistance by the addition of Nb, Mo, or the like. Patent Literature 4 discloses an invention relating to a disc material having excellent heat resistance in which a quenching treatment is performed by quenching from a temperature higher than 1,000° C. Patent Literature 5 discloses, as a brake disc having excellent temper softening resistance, a brake disc having a martensite structure in which a prior austenite grain has an average grain size of 8 μm or more, and Patent Literature 6 discloses an invention in which martensite accounts for 75% or more of an area ratio of a quenched structure and the Nb content is 0.10% or more and 0.60% or less.